The present invention relates to a reflection type liquid crystal display device using a light scattering type liquid crystal display element which is capable of displaying bright and beautiful colors such as white as bright as paper white and of operating when being driven at a low voltage and with a small current, and which can be used, for example, in a clock, a mobile telephone or a mobile information terminal.
The liquid crystal display device is frequently used as a display panel in devices for various applications because it has excellent features such as a small thickness and a low power consumption.
The display modes of the liquid crystal display element are exemplified by one which makes use of the double refraction or the optical rotary power of the liquid crystal with one or two polarizing plates, as represented by the TN (Twisted Nematic) mode or the STN (Super Twisted Nematic) mode. The efficiency of using the light in the TN mode or in the STN mode is theoretically 50% or less to thereby present a dark display because of a light absorption loss by a polarizing plate.
There is another display system not using a polarizing plate instead but the light scattering characteristics of the liquid crystal, as represented by the phase conversion mode and the polymer dispersed mode. These light scattering modes need no polarizing plate and are thus without the optical absorption loss of the polarizing plate so that they can make effective use of the light to provide a bright display.
The polymer dispersed type liquid crystal display panel includes a multi-layered construction in which the liquid crystal material and the polymer material are phase-separated to disperse the liquid crystal in small droplets or in a minute continuous phase in the polymer layer, so that it is enabled to display an image by switching the scattering state and the transparent state in dependence on the existence of an electric field. The polymer dispersed type liquid crystal panel is disclosed in Japanese Patent Publication No. 52843/1991 or Japanese Patent Laid-Open No. 501512/1988 or 6150218/1986, for example.
As a means for coloring the light scattering mode liquid crystal panel, there has been proposed a method of arranging various mirrors or optical absorbing plates. At the back of the dynamic scattering mode light modulation layer, there are arranged a non-metallic multi-layered thin film (e.g., an interference filter) and a low lightness background substrate (e.g., a colored background plate), as disclosed in Japanese Patent Laid-Open No. 20749/1975. In Japanese Patent Laid-Open No. 10924/1984, likewise, a dichroic mirror and a bright color scattering reflection plate are arranged at the back of the phase conversion type or dynamic scattering mode display layer. In Japanese Patent Laid-Open No. 152029/1995, moreover, a color filter is arranged on the front face of a polymer dispersing type liquid crystal layer, and a selective reflection layer (e.g., an interference filter) and a scattering reflection layer for reflecting a white color or a color transmitted through the selective reflection layer are arranged at the back of the liquid crystal layer. According to the means common among those techniques, the non-metallic multi-layered thin film and the so-called "interference filter" such as the dichroic mirror or the selective reflection layer are arranged at the back of the light scattering mode display element whereas the scattering reflection plate such as paper or pain is arranged at the back of the interference filter, and the color filter is arranged in front of the light scattering mode display element. Moreover, the light scattering mode display element is exemplified by the liquid crystal in the light scattering mode such as the dynamic scattering mode (DSM), the phase conversion mode or the polymer dispersed mode.
However, the display device, as disclosed in Japanese Patent Laid-Open No. 20749/1975, 10924/1984 or 152029/1995, has the following defects.
Firstly, in the liquid crystal display device of the prior art, the liquid crystal of the light scattering mode such as the dynamic scattering mode (DSM), the phase conversion mode (PCM) or the polymer dispersed mode is used as the light scattering mode display element.
The dynamic scattering mode liquid crystal display device has problems in that it is driven by an electric current as much as by 10 to 100 times higher than the general TN mode or STN mode and that the reliability of the dynamic scattering mode itself is poor. Therefore, the dynamic scattering mode liquid crystal display device of the prior art is hard to mount in equipment especially of the type stressing high battery lifetime such as a watch or small-sized mobile equipment.
The phase conversion mode liquid crystal display device may use the thermal opaqueness for the scattering state. This thermal opaqueness is seriously deteriorated by the influence of external stress and the heat cycle. This problem makes it difficult to mount and use the phase conversion mode liquid crystal display device of the prior art in a small-sized mobile device such as a watch.
The polymer dispersed mode liquid crystal display device has been exemplified in the prior art by the liquid crystal display element having the micro-capsuled structure, in which a low-molecule nematic liquid crystal is confined in a high-molecule micro capsule, or the liquid crystal display element having the polymer matrix structure in which a low-molecule nematic liquid crystal is confined in a porous polymer matrix. In a polymer dispersed type liquid crystal display element having a structure in which the liquid crystal droplets are arranged in those polymers, however, the liquid crystal molecules are arranged along the polymer wall faces of the liquid crystal droplets so that the liquid crystal molecules in the vicinity of the polymer wall faces are seriously influenced by the polymer wall faces. As a result, the liquid crystal molecules cannot be easily arrayed, even if a voltage is applied, in the direction of the electric field but at an angle. For an obliquely incident light, therefore, an inconsistency occurs between the refractive index in the longitudinal direction of the liquid crystal molecules and the refractive index of the polymer so that a slight scattering cannot be avoided. This raises a problem in that an excellently transparent state cannot be achieved unless an extremely high voltage is applied. If the cell gap is increased to raise the scattering characteristics, moreover, the obliquely incident light is liable to be slightly scattered when the voltage is applied, and the display color turns opaque so that a clear color cannot be reconstructed to fail to retain the contrast. In the droplet type polymer dispersed type liquid crystal display element, on the other hand, the method of lowering the drive voltage is exemplified by reducing the cell gap. If the cell gap is reduced to 10 microns or less, for example, the scattering state is seriously deteriorated to fail to reach the practical level. This makes it difficult for the droplet type polymer dispersed type liquid crystal display element to make a sufficient scattered state and transparent state compatible and to realize an excellent contrast. This makes it difficult for the droplet type polymer scattered type liquid crystal display element of the prior art to be mounted and practiced in a small-sized mobile device of low voltage and power consumption type such as the watch.
Secondly, in the liquid crystal display device having a structure in which the liquid crystal display element of the aforementioned light scattering mode of the prior art such as the dynamic scattering mode (DSM), the phase conversion mode (PCM) or the droplet type polymer scattered mode is used as a light modulation layer and in which an interference filter and a scattering reflection layer are arranged at the back, the light scattering layer has weak scattering characteristics so that the light having passed through the color separation layer is scattered and reflected, when no voltage is applied (i.e., in the scattering state), by the scattering/reflecting layer. Because of a serious influence of the scattered and reflected light, the region in the scattering state is colored to the same color as that of the light having passed through the color separation layer. In short, there arises a problem in that a color as white as paper white cannot be realized, namely, that the background which should be intrinsically white is colored. Another problem is the dependency of color on the visual sense.
In the case of the reflection type liquid crystal display device having the structure in which the droplet type polymer dispersed type liquid crystal layer is used as the light modulation layer and in which the interference filter and the mirror layer as the reflection layer are arranged at the back, with no application of voltage (i.e., in the transparent state), the liquid crystal molecules can be arrayed not in the field direction but at an angle to cause an inconsistency between the longitudinal refractive index of the liquid crystal molecules and the refractive index of the polymer thereby to leave a slight dispersion. As a result, the regularly reflected light is diffused with a width to raise a problem in that the invisible range is widened, and the diffused color is mixed with the color having passed through the color separation layer to raise the problem that the color purity drops.
Thirdly, in the case of the reflection liquid crystal display device, because of an unnecessary reflection of an ambient light on the liquid crystal panel surface, there arises a problem in that the displayed content is hard to view and that the use efficiency of the ambient light drops.
Moreover, the ambient light contains an ultraviolet ray so that the liquid crystal display device of the light scattering mode of the prior art, as weak to the ultraviolet ray, is troubled by a problem in that the light modulation layer is deteriorated by the ultraviolet ray.